A semiconductor device including a data storage pattern is provided. The semiconductor device includes a first conductive line disposed on a substrate and extending in a first direction, a second conductive line disposed on the first conductive line and extending in a second direction, and a first data storage structure and a first selector structure disposed between the first conductive line and the second conductive line and connected in series. The first data storage structure includes a first lower data storage electrode, a first data storage pattern, and a first upper data storage electrode. The first lower data storage electrode includes a first portion facing the first upper data storage electrode and vertically aligned with the first upper data storage electrode. The first data storage pattern includes a first side surface and a second side surface facing each other. The first upper data storage electrode and the first portion of the first lower data storage electrode are disposed to be closer to the first side surface of the first data storage pattern than to the second side surface of the first data storage pattern.